Shoreline Changes on the Wave-Influenced Senegal River

water

Article Shoreline Changes on the Wave-Inﬂuenced Senegal River Delta, West Africa: The Roles of Natural Processes and Human Interventions

Mamadou Sadio 1,2, Edward J. Anthony 1,*, Amadou Tahirou Diaw 2, Philippe Dussouillez 1, Jules T. Fleury 1, Alioune Kane 3, Rafael Almar 4 and Elodie Kestenare 4

1 Aix-Marseille University, CEREGE UM 34, Europôle de l’Arbois, 13545 Aix en Provence Cedex 04, France; [email protected] (M.S.); [email protected] (P.D.); ﬂ[email protected] (J.T.F.) 2 Laboratoire d’Enseignement et de Recherche en Géomatique, Ecole Supérieure Polytechnique, Université Cheikh Anta Diop, Dakar, Sénégal; [email protected] 3 Laboratoire de Morphologie et d’Hydrologie, Université Cheikh Anta Diop, Dakar, Sénégal; [email protected] 4 LEGOS (CNRS-IRD-CNES-University of Toulouse), 31400 Toulouse, France; [email protected] (R.A.); [email protected] (E.K.) * Correspondence: [email protected]

Academic Editors: Sylvain Ouillon and John W. Day Received: 24 December 2016; Accepted: 12 May 2017; Published: 19 May 2017

Abstract: The Senegal River delta in West Africa, one of the finest examples of “wave-influenced” deltas, is bounded by a spit periodically breached by waves, each breach then acting as a shifting mouth of the Senegal River. Using European Re-Analysis (ERA) hindcast wave data from 1984 to 2015 generated by the Wave Atmospheric Model (WAM) of the European Centre for Medium-Range Weather Forecasts (ECMWF), we calculated longshore sediment transport rates along the spit. We also analysed spit width, spit migration rates, and changes in the position and width of the river mouth from aerial photographs and satellite images between 1954 and 2015. In 2003, an artificial breach was cut through the spit to prevent river flooding of the historic city of St. Louis. Analysis of past spit growth rates and of the breaching length scale associated with maximum spit elongation, and a reported increase in the frequency of high flood water levels between 1994 and 2003, suggest, together, that an impending natural breach was likely to have occurred close to the time frame of the artificial 2003 breach. Following this breach, the new river mouth was widened rapidly by flood discharge evacuation, but stabilised to its usual hydraulic width of <2 km. In 2012, severe erosion of the residual spit downdrift of the mouth may have been due to a significant drop (~15%) in the longshore sand transport volume and to a lower sediment bypassing fraction across the river mouth. This wave erosion of the residual spit led to rapid exceptional widening of the mouth to ~5 km that has not been compensated by updrift spit elongation. This wider mouth may now be acting as a large depocentre for sand transported alongshore from updrift, and has contributed to an increase in the tidal influence affecting the lower delta. Wave erosion of the residual spit has led to the destruction of villages, tourist facilities and infrastructure. This erosion of the spit has also exposed part of the delta plain directly to waves, and reinforced the saline intrusion within the Senegal delta. Understanding the mechanisms and processes behind these changes is important in planning of future shoreline management and decision-making regarding the articulations between coastal protection offered by the wave-built spit and flooding of the lower delta plain of the Senegal River.

Keywords: Senegal River delta; Langue de Barbarie spit; delta vulnerability; river-mouth migration; spit breaching; ERA hindcast waves; longshore sediment transport

Water 2017, 9, 357; doi:10.3390/w9050357 www.mdpi.com/journal/water Water 2017, 9, 357 2 of 17

1. Introduction The impacts of human activities on coasts are often accompanied by a lack of understanding of the consequences of these activities on the hydrodynamic and sediment redistribution processes that shape coasts [1,2]. Alongshore sediment transport, gradients in transport, and interception of drifting sediment by natural or artificial (man-made) boundaries, including river mouths and inlets, are, from a coastal management point of view, very important, as these processes are significant drivers of short-term to medium-term (days to years) shoreline change. Much of the West African coast (Figure1) is wave-dominated, and is classified as a cyclone- and storm-free “West Coast Swell Environment” in the global wave classification scheme of Davies [3], with a subsidiary contribution from shorter-period trade-wind waves from the Atlantic. In Figure1, continental shelf width (clearer hue along the coast) is a fine indicator of the distribution of long stretches of wave-dominated coast (narrow shelf) and the much more limited, predominantly tidal, estuarine sector between Sierra Leone and Guinea-Bissau (broad, low-gradient shelf), subject to significant wave energy dissipation [4,5]. The West African coast is also characterised by a plethora of river deltas, the largest of which are those of the Niger, Senegal and Volta (Figure1). Abundant sand supplies and strong wave-induced longshore drift have favoured the construction of numerous sand barriers, including at the mouths of these three deltas. These barriers are major settlement sites on the coast as they provide higher-lying areas above lagoons and wetlands, while acting as valuable aquifers. On the coast of Senegal, the barriers are generally elongate to curvilinear spits formed at the mouths of tidal or fluvial ria-like embayments. These spits are commonly capped by dunes, but individual beach ridges are visible in some of the more southern ones. These spits have a protective role on the back-barrier wetlands and lagoons by buffering wave energy. By forming alongshore barriers, they are also important in the regulation of the freshwater-saltwater balance and ecology of these lagoons and back-barrier wetlands, both of which can be considerably altered by breaches in the spits or by spit erosion [6,7]. This is particularly the case of the largest of the Senegal wetland systems, that of the lower Senegal River and delta plain (Figure2). The Senegal River delta is an iconic example of a delta subject to strong wave action [8–12]. This delta is often represented in the ternary (river-wave-tide) classification of Galloway [13] at the wave apex. Using a fluvial dominance ratio—defined as river sediment input versus the potential maximum alongshore sediment transport away from the delta mouth—to quantify the balance between river inputs and the ability of waves to spread sediments along the coast, Nienhuis et al. [12] computed a value of 0.04 for the Senegal, which highlights the strong role of wave-induced longshore transport along this delta’s shoreline. A manifestation of this strong longshore transport potential is a long narrow sand spit presently fronting the delta plain, the Langue de Barbarie [10]. This spit has historically played an important role not only in the protection of the lower Senegal delta plain but also in regulating saltwater intrusion by diverting the mouth of the river several kilometres southwards. Of particular significance, in terms of long-term flood-risk and coastal management, is the historic and picturesque city of St. Louis (population in 2013: 300,000), a UNESCO world heritage site located in the proximal part of the delta (Figure2). The cultural attractiveness of St. Louis, a French colonial city, and the biodiversity of the deltaic wetlands and lagoon bound by the Langue de Barbarie spit have also generated a substantial rise in tourism. Much of St. Louis, which has undergone a rapid growth in population over the last 50 years, lies at an elevation of less than 2.5 m above sea level [14], and the city has, therefore, been prone to the flooding that affects the lower Senegal valley in the rainy season (May to October). In October 2003, to avoid flooding of St. Louis in the wake of a massive rise in the water discharge of the Senegal River, an artificial breach was hastily cut through the Langue de Barbarie, generating rapid reworking of the spit. In the present paper, we describe the recent dynamics of the spit within the framework of development of the Senegal delta and specifically aim at disentangling processes of natural forcing from those of the impact of this breach. Two approaches are used in the study: (1) clearly define the wave climate and longshore sediment transport potential along the Langue de Barbarie; and (2) compare spit behaviour patterns prior to, and following the October 2003 artificial Water 2017, 9, 357 3 of 17 breach.Water 2017 Both, 9, 357 of these approaches are important in understanding the current dynamics prevailing3 of 17 Water 2017, 9, 357 3 of 17 alongprevailing this deltaic along coast. this Theydeltaic should coast. alsoThey be should of use also in planning be of use of futurein planning shoreline of future management shoreline and prevailing along this deltaic coast. They should also be of use in planning of future shoreline decision-makingmanagement and regarding decision-making the articulations regarding between the articulations coastal protection between coastal offered protection by the wave-built offered by spit management and decision-making regarding the articulations between coastal protection offered by andthe flooding wave-built of the spit lower and flooding delta plain of the of lower the Senegal delta plain River. of the Senegal River. the wave-built spit and flooding of the lower delta plain of the Senegal River.

Figure 1. The coast of West Africa, showing the Senegal (box) and other major river deltas. Much of FigureFigure 1. The1. The coast coast of of West West Africa, Africa, showing showing thethe SenegalSenegal (box) and and other other major major river river deltas. deltas. Much Much of of this coast is wave-dominated, and is characterised by beach-ridge sand barriers and spits. thisthis coast coast is wave-dominated,is wave-dominated, and and is is characterised characterised byby beach-ridge sand sand barriers barriers and and spits. spits.

Figure 2. The Senegal River delta, a fine example of a wave-dominated delta characterised by the Figure 2. The Senegal River delta, a fine example of a wave-dominated delta characterised by the FigureLangue 2. Thede Barbarie Senegal spit River and delta,a river-mouth a ﬁne example system subject of a wave-dominated to strong north-south delta longshore characterised drift. by the LangueLangue de de Barbarie Barbarie spit spit and and a a river-mouth river-mouth systemsystem subjectsubject to strong north-south north-south longshore longshore drift. drift. 2. The Senegal River and Delta 2. The Senegal River and Delta 2. The SenegalThe Senegal River River and is Deltaabout 1800 km long, and is the second longest river in West Africa after the The Senegal River is about 1800 km long, and is the second longest river in West Africa after the Niger.The SenegalThe catchment River is size about has 1800 been km estimated long, and at is345,000 the second km2 longest[15], much river of in it Westcovering Africa the after arid the Niger. The catchment size has been estimated at 345,000 km2 [15], much of it covering the arid western Sahel. The river’s discharge has been particularly2 affected by Sahelian droughts since the Niger.western The Sahel. catchment The sizeriver’s has discharge been estimated has been at 345,000particularly km affected[15], much by ofSahelian it covering droughts the arid since western the 1970s [16]. The mean annual water discharge at Bakel, the reference station of the Senegal River, Sahel.1970s The [16]. river’s The mean discharge annual has water been discharge particularly at Bakel, affected the by reference Sahelian station droughts of the since Senegal the 1970s River, [16 ]. situated 557 km upstream of St. Louis, is 676 m3/s, and varies from a mean low dry season value of Thesituated mean annual557 km waterupstream discharge of St. Louis, at Bakel, is 676 the m reference3/s, and varies station from of the a mean Senegal low River, dry season situated value 557 of km 10 m3/s in May, to a mean maximum3 flood value of 3320 m3/s in September at the height of 3the rainy upstream10 m3/s in of May, St. Louis, to a mean is 676 maximum m /s, and flood varies value from of 3320 a mean m3/s low in September dry season at value the height of 10 of m the/s rainy in May, season [17]. The interannual variability is extremely3 high, with a mean annual discharge ranging toseason a mean [17]. maximum The interannual ﬂood value variability of 3320 is m extremely/s in September high, with at a the mean height annual of the discharge rainy season ranging [17 ]. from 250 to 1400 m3/s. Little is known of the solid discharge of the Senegal. This solid load has been Thefrom interannual 250 to 1400 variability m3/s. Little is is extremely known of high,the solid with discharge a mean of annual the Senegal. discharge This solid ranging load from has been 250 to estimated at 0.9 to 1 × 106 tonnes a year [18], a rather low figure when viewed against the size of the estimated at 0.9 to 1 × 106 tonnes a year [18], a rather low figure when viewed against the size of the

Water 2017, 9, 357 4 of 17

1400 m3/s. Little is known of the solid discharge of the Senegal. This solid load has been estimated at 0.9 to 1 × 106 tonnes a year [18], a rather low figure when viewed against the size of the river’s catchment and when compared to other tropical rivers. The solid discharge is largely dominated by suspended load transport [19]. The lower Senegal delta is characterised by high biological productivity and by rich agricultural and fishing sectors. In November 1985, the Diama dam (Figure2) was built in the lower river valley 23 km upstream from St. Louis. The dam was commissioned with the twin aims of preventing saltwater intrusion, which, hitherto, penetrated up to 350 km upstream in the lower Senegal valley, and regulating the river’s rainy season discharge in order to improve irrigation of agricultural lands [15]. The delta plain provides 8% of the arable land of Senegal [20]. The Senegal delta coast is fronted by a relatively narrow continental shelf only 15–20 km wide. The dominant waves are from the northwest, and this direction is especially prevalent during the dry season from November to June. One of the objectives of this study is to highlight the salient characteristics of this “West Coast” wave setting (see Results). The tidal regime along the Langue de Barbarie is semi-diurnal and the range microtidal, comprised between 0.5 m at neap tides and 1.6 m at spring tides. The relatively moderate river discharge, including during the flood season, the permanence of moderately energetic waves propagating across a relatively narrow shelf, and the microtidal regime, are three conditions that have been forwarded to explain the wave-dominated character of the Senegal River delta [10]. The stratigraphy and patterns of Holocene geomorphic development of the Senegal delta have been highlighted from borehole data, limited radiocarbon dating, and analysis of plan-view sand barrier and longshore drift patterns in relation to the courses of the river [21,22]. The delta plain prograded as a bayhead delta within a confined setting rich in Late Pleistocene aeolian deposits (Ogolian dunes) that extended as subaerial forms over the then exposed shelf during the last lowstand that peaked at 19,000 year B.P. [21,22]. Mud supplied by the river and fine sand derived from reworking of dunes inland by river-channel meandering have generated up to 8.5 km of essentially fine-grained delta-plain progradation within this bayhead setting. Although the delta plain does not protrude significantly into the Atlantic Ocean (Figure2), probably because of the combination of this embayed setting and the relatively steep narrow shelf, the Senegal has, nevertheless, formed quite a large delta with an area of about 4254 km2, much of which is subaerial, the ratio of the subaerial to subaqueous delta being 2:1 [9]. This mud-rich delta plain is bound by massive sandy barriers [21] built by waves propagating over loose aeolian deposits on the submerged narrow shelf. These coarse-grained barriers are separated by swales comprising abandoned river courses. Efficient trapping of river-borne sediments by the aggrading delta plain behind these wave-built sand barriers probably explains the high subaerial-subaqueous ratio of this delta, which is also consistent with the limited delta bulge compared to the more cuspate form commonly evinced by wave-dominated deltas. Remnants of these degraded barriers with beach ridges are discernible within the outer margins of the delta plain south of St. Louis. These spits are ancestral to the present Langue de Barbarie spit. Michel [21] dated the formation of these barriers at between 4000 and 1900 B.P. In essence, therefore, much of the Holocene development of the Senegal delta has consisted in embayment infilling behind the protection of these sand barriers, thus potentially giving rise to two distinct facies arrangements: wave-built sand bodies and back-barrier embayment facies represented by infilling fluvial deposits, including fine sands reworked from the Ogolian dunes by river meandering. We used European Re-Analysis (ERA) hindcast wave data from 1984 to 2015 generated by the ECMWF Wave Atmospheric Model to characterise the wave climate affecting the Senegal River delta and to calculate longshore sediment transport rates along the spit. We then analysed changes in the position of the river mouth, rates of spit migration and spit width from aerial photographs and satellite images between 1954 and 2015 in order to characterise the shoreline morphodynamic context of the delta (see Materials and Methods). Water 2017, 9, 357 5 of 17 Water 2017, 9, 357 5 of 17

3. Results 3. Results 3.1. Wave Climate and Alongshore Sediment Transport 3.1. Wave Climate and Alongshore Sediment Transport The wave climate of the Senegal delta shoreline is characterised by two components with stronglyThe wave contrasting climate ofbehaviour: the Senegal wind delta waves shoreline generated is characterised locally and bya dominant two components component with of strongly long contrastingswell waves behaviour: from mid- wind to waveshigh latitudes generated (Figure locally 3). and The a dominantregion is not component directly ofaffected long swell by major waves fromstorms mid- or to cyclones high latitudes but the (Figure influence3). The of regionthese dist is notant directlyhigh-energy affected events by majorin the stormsNorth Atlantic or cyclones is butmaterialised the inﬂuence in of the these wave distant climate. high-energy Averaging events over inthe the 1984–2015 North Atlantic period isgives materialised annual significant in the wave climate.swell Averagingand wind wave over he theights 1984–2015 respectively period of givesHs = 1.52 annual m and signiﬁcant 0.53, and swell peak andswell wind and wavewind heightswave respectivelyperiods of ofTpH =s 9.23= 1.52 s and mand 3.060.53, s. The and dominant peak swell swell and waves wind originate wave periods from WNW of Tp =to 9.23 N and s and have 3.06 a s. Themean dominant direction swell of waves 325°. originate The direction from WNWgraph to (b Nottom, and have Figure a mean 3) shows direction a brief of 325 August◦. The directionswing graphdominated (bottom, by Figureswell waves3) shows from a the brief south. August Wind swing waves dominatedshow a much by wider swell directional waves from window the south. and Winda mean waves of show295°. There a much is widera clear directional seasonal modulation, window and swell a mean activity of 295peaking◦. There during is a the clear northern seasonal modulation,hemisphere swell winter activity with strong peaking stor duringm activity the northernat mid to hemispherehigh latitude winters. Wind with waves strong also stormshow larger activity at midday-to-day to high and latitudes. monthly Wind variability. waves alsoContrary show to larger swell day-to-daywaves, these and wind monthly waves variability.are driven by Contrary local to swelltropical waves, winds these and windshow waves peaks arein drivenspring byand local autumn tropical that winds correspond and show to the peaks passages in spring of the and autumnIntertropical that correspond Convergence to the Zone passages over Senegal. of the Intertropical Convergence Zone over Senegal.

FigureFigure 3. Mean3. Mean wave wave characteristics characteristics (signiﬁcant (significant wave wave height height (Hs), ( peakHs), wavepeak wave period period (T), and (T incident), and ◦ directionincident ( ))direction along the (°)) Senegal along the River Senegal delta coastRiver fromdelta 1984 coast to from 2015 1984 ERA to hindcast 2015 ERA data. hindcast Orange: data. swell waves,Orange: blue: swell wind waves, waves. blue: wind waves.

AsAs both both swell swell and and wind wind waves waves originate originate dominantlydominantly from W W to to N, N, this this results results in in an an oblique oblique approachapproach to to the the coastline coastline that that generates generates aa largelarge longshorelongshore sediment transport transport (LST) (LST) towards towards the the south. Figure 4 depicts the annual LST along the Senegal delta coast for swell waves and wind waves south. Figure4 depicts the annual LST along the Senegal delta coast for swell waves and wind waves computed using the formula of Kaczmarek et al. [23] as described in the Methods Section. The mean computed using the formula of Kaczmarek et al. [23] as described in the Methods Section. The mean annual net transport induced by swell waves over the 32-year period of the ERA dataset is of the annual net transport induced by swell waves over the 32-year period of the ERA dataset is of the order order of 669 × 103 m3/year, i.e., ~89% of the total transport, the total wind-wave-induced LST of 669 × 103 m3/year, i.e., ~89% of the total transport, the total wind-wave-induced LST amounting amounting to only 80 × 103 m3/year. LST is very largely dominated by southwards swell-induced × 3 3 to onlydrift 80which10 amountsm /year. to an LST annual is very mean largely of 611 dominated× 103 m3/year, by while southwards net wind-wave-induced swell-induced drifttransport which × 3 3 amountsin the tosame an annualdirection mean is only of 611 59 × 10103 mm3/year./year, Counter while netLST wind-wave-induced towards the north is transport nearly an in order the same of 3 3 directionmagnitude is only less: 59 58× × 10103 m3/year/year. for Counter swell waves LST and towards 21 × 10 the3 m north3/year isfor nearly wind anwaves, order i.e., of only magnitude ~14% 3 3 3 3 less:of 58the× total10 m LST./year These for swellcomputed waves sediment and 21 ×tran10sportm /year volumes for windare remarkably waves, i.e., similar only ~14% to those of the total LST. These computed sediment transport volumes are remarkably similar to those provided by

Water 2017, 9, 357 6 of 17

provided by the French engineering firm [24] SOGREAH (1994) who calculated a drift volume that decreases from north to south along the spit from 700 to 600 × 103 m3/year.

3.2. LST and Growth Dynamics of the Langue de Barbarie Spit The Langue de Barbarie spit is a product of the strong wave action and high LST that have controlled the morphosedimentary development of the seaward fringe of the Senegal River delta (Figure 2). These observations and the satellite data also provide insight on the sand sourcing the seaward face of the Langue de Barbarie, which is derived from the coast and shoreface of Mauritania updrift of the historic mouth of the Senegal (Figure 1), in agreement with a conclusion also reached by Barusseau et al. [25]. The satellite data show that the Langue de Barbarie spit is a 100–400 m-wide feature. The spit is capped by aeolian dunes 5–10 m high. Widening of the spit and dune accretion occur through abstraction of the large alongshore sediment supply, especially in the distal section where bare, unvegetated dunes prevail, as well as through distal spit extension [26]. In contrast, the proximal sector, near St. Louis has been characterised by a much more stabilised dune system. Since 1900, a major coastal management preoccupation in the lower Senegal delta has been that of preventing natural breaches in the Langue de Barbarie in the vicinity of St. Louis, as this posed a threat for developing tourist facilities and infrastructure on the spit downdrift of every breach. Spit protection was achieved through the fixing and consolidation of the aeolian dunes via plantations of Filao (Casuarina equisetifolia) [27]. The alongshore transport volume would appear to undergo increasingly larger aeolian dune trapping of sand in the relatively poorly vegetated distal zone, compared to the relatively more urbanised and vegetated proximal sector of St. Louis. The former zone also represents one of active remigration following past natural breaches. The longshore gradient in sediment transport highlighted by SOGREAH [24] would appear to correspond to these morphological variations as one goes from the proximal to the distal sector of the spit. The successive locations of the mouth of the Senegal River have been controlled by spit breaching followed by downdrift spit elongation. Spit breaching has generally been caused by increases in river water level, especially over the narrowest and lowest parts of the spit [26]. Once breaching occurs, the new breach is exploited by river discharge, tidal ingression, and waves, and forms a new river mouth. This leads to the older mouth becoming underfit and sealed by distal spit attachment to the shore. Natural breaching is attended by spit elongation through the classic Waterformation2017, 9, 357 of dune-capped beach ridges at the distal end, and this process has undoubtedly been6 of 17 favoured by the shallow overall depths of the mouth (2.5–3.5 m according to Bâ et al. [28]). The mouth is characterised by bars and spit recurves, remnants of which are identified in updrift thelocations French engineeringon the spit. The ﬁrm mouth [24] SOGREAH bars apparently (1994) se whorve as calculated platforms a for drift spit volume extension that and decreases eventual from northriver-mouth to south diversion along the southwards. spit from 700 to 600 × 103 m3/year.

Figure 4. Gross annual longshore sediment transport (LST) along the Senegal River delta coast from Figure 4. Gross annual longshore sediment transport (LST) along the Senegal River delta coast from 1984 to 2015. Orange: swell waves, blue: wind waves. Note the significant drop in swell-induced LST 1984 to 2015. Orange: swell waves, blue: wind waves. Note the significant drop in swell-induced LST between 2009 and 2012, corresponding to a decrease of >35%, and the sharp rise the following year. between 2009 and 2012, corresponding to a decrease of >35%, and the sharp rise the following year. 3.3. Historical and Recent Changes of the Langue de Barbarie Spit Prior to the 2003 Artificial Breach 3.2. LST and Growth Dynamics of the Langue de Barbarie Spit Joiré [29] and Tricart [30] situated the mouth of the river in the vicinity of St. Louis at about the mid-17thThe Langue century, de while Barbarie a historical spit is analysis a product of spit of mobility the strong and wave of the action associated and locations high LST of thatmouth have controlledopenings thedocumented morphosedimentary even earlier developmentmouth scars no ofrth the of seaward St. Louis fringe [27]. ofThe the Langue Senegal de RiverBarbarie delta (Figure2). These observations and the satellite data also provide insight on the sand sourcing the seaward face of the Langue de Barbarie, which is derived from the coast and shoreface of Mauritania updrift of the historic mouth of the Senegal (Figure1), in agreement with a conclusion also reached by Barusseau et al. [25]. The satellite data show that the Langue de Barbarie spit is a 100–400 m-wide feature. The spit is capped by aeolian dunes 5–10 m high. Widening of the spit and dune accretion occur through abstraction of the large alongshore sediment supply, especially in the distal section where bare, unvegetated dunes prevail, as well as through distal spit extension [26]. In contrast, the proximal sector, near St. Louis has been characterised by a much more stabilised dune system. Since 1900, a major coastal management preoccupation in the lower Senegal delta has been that of preventing natural breaches in the Langue de Barbarie in the vicinity of St. Louis, as this posed a threat for developing tourist facilities and infrastructure on the spit downdrift of every breach. Spit protection was achieved through the fixing and consolidation of the aeolian dunes via plantations of Filao (Casuarina equisetifolia)[27]. The alongshore transport volume would appear to undergo increasingly larger aeolian dune trapping of sand in the relatively poorly vegetated distal zone, compared to the relatively more urbanised and vegetated proximal sector of St. Louis. The former zone also represents one of active remigration following past natural breaches. The longshore gradient in sediment transport highlighted by SOGREAH [24] would appear to correspond to these morphological variations as one goes from the proximal to the distal sector of the spit. The successive locations of the mouth of the Senegal River have been controlled by spit breaching followed by downdrift spit elongation. Spit breaching has generally been caused by increases in river water level, especially over the narrowest and lowest parts of the spit [26]. Once breaching occurs, the new breach is exploited by river discharge, tidal ingression, and waves, and forms a new river mouth. This leads to the older mouth becoming underfit and sealed by distal spit attachment to the shore. Natural breaching is attended by spit elongation through the classic formation of dune-capped beach ridges at the distal end, and this process has undoubtedly been favoured by the shallow overall depths of the mouth (2.5–3.5 m according to Bâ et al. [28]). The mouth is characterised by bars and spit recurves, remnants of which are identified in updrift locations on the spit. The mouth bars apparently serve as platforms for spit extension and eventual river-mouth diversion southwards.

3.3. Historical and Recent Changes of the Langue de Barbarie Spit Prior to the 2003 Artificial Breach Joiré [29] and Tricart [30] situated the mouth of the river in the vicinity of St. Louis at about the mid-17th century, while a historical analysis of spit mobility and of the associated locations of Water 2017, 9, 357 7 of 17 mouthWater openings 2017, 9, 357 documented even earlier mouth scars north of St. Louis [27]. The Langue de7 Barbarie of 17 lengthenedlengthened by 11by km 11 km between between 1850 1850 and and 1900 1900 (about (about 220 220 m am year), a year), with with a distal a distal tip tip located located 15 15 km km south of St.south Louis of atSt. theLouis turn at the ofthe turn 20th of the century, 20th century, and the an spitd the was spit affected was affected over over this this 50-year 50-year period period by by seven breachesseven [27breaches]. Between [27]. 1900Between and 1973,1900 and 13 other 1973, breaches 13 other occurredbreaches acrossoccurred the across Langue the de Langue Barbarie de [27], thusBarbarie suggesting [27], athus breaching suggesting timescale a breaching (see timescale Nienhuis (see et al.Nienhuis [31]) of et ~6 al. years. [31]) of There ~6 years. were There no breacheswere betweenno breaches 1973 and between 2003. 1973 and 2003. FollowingFollowing the the 1973 1973 breach, breach, the the Langue Langue dede Barbarie lengthened lengthened by by 12.5 12.5 km km (at (ata mean a mean raterate of of ~400~400 m/year) m/year) before before the spitthe spit was was artificially artificially breached breached in 2003. in 2003. Spit elongationSpit elongation calculated calculated from from satellite images,satellite aerial images, photographs aerial photographs and field measurements and field measurements has, however, has, however, fluctuated fluctuated widely fromwidely low from values low values of nearly nil to <170 m/year (1985–1986, 1990–1991) to >1200 m/year (1987–1989, of nearly nil to <170 m/year (1985–1986, 1990–1991) to >1200 m/year (1987–1989, 2000–2002) (Figure5). 2000–2002) (Figure 5). Gac et al. [27] showed that the farthest downdrift position of the mouth of the Gac et al. [27] showed that the farthest downdrift position of the mouth of the river, which corresponds river, which corresponds to the maximal distal spit extension, did not exceed 30 km over the 80-year to theperiod maximal covered distal by spittheir extension, observations, did which not exceed is close 30 to km a overvalue the of 80-year28 km reported period coveredin an earlier by their observations,study [32]. whichThe successive is close tolocations a value of of the 28 kmmout reportedh of the inSenegal an earlier River study since [1973,32]. Thewhich successive also locationscorrespond of the mouthto those of of the the Senegal distal Rivertip of sincethe so 1973,uthward-extending which also correspond spit, are toshown those in of Figure the distal 5, tip of thealongside southward-extending the migration rates. spit, The are migration shown in between Figure5 1973, alongside and 2003 the brought migration the distal rates. tip The of the migration spit betweenclose 1973to the and maximum 2003 brought spit length. the distalThe data tip from of the satellite spit close images to the show maximum a relatively spit narrow length. mouth The data from(0.25–<1 satellite km-wide) images showwith athe relatively exception narrow of the mouthyears 1968–1973 (0.25–<1 km-wide)and 1988–1989 with thewhen exception the width of the yearsexceeded 1968–1973 1.5 km and (Figure 1988–1989 6). when the width exceeded 1.5 km (Figure6).

FigureFigure 5. Successive 5. Successive dated dated locations locations of of the the mouthmouth of the Senegal Senegal River River delta delta materialised materialised by the by thedistal distal tip oftip the of the Langue Langue de de Barbarie Barbarie spit spit (left (left);); and and spit spit migrationmigration rates rates in in m/year m/year from from 1968 1968 to 2004 to 2004 (right (right). ).

Water 2017, 9, 357 8 of 17

FigureFigure 6. 6.Width Width of of the the mouth mouth of of the the Senegal Senegal RiverRiver delta between 1954 1954 and and 2015. 2015. Except Except for for the the years years 1968–19731968–1973 and and 1987–1988, 1987–1988, the the width width did notdid exceednot exceed 1 km, 1 prior km, toprior the 2003to the artificial 2003 artificial breach. Followingbreach. thisFollowing breach, the this width breach, of thethe mouthwidth of fluctuated the mouth to fluc attaintuated ~1 km to attain in 2008, ~1 whichkm in corresponds2008, which corresponds to the average to the average width of the “fluvial” river mouth. A further rapid increase, not related to river-mouth width of the “fluvial” river mouth. A further rapid increase, not related to river-mouth hydraulics hydraulics (see Discussion), occurred thereafter, peaking in 2013. (see Discussion), occurred thereafter, peaking in 2013. 3.4. The Artificial Breach in 2003 and Post-Breach Spit and River-Mouth Evolution 3.4. The Artificial Breach in 2003 and Post-Breach Spit and River-Mouth Evolution An emergency water level in St. Louis prompted artificial breaching, on the night of 3 October 2003,An of emergency the Langue water de Barbarie level in in St. the Louis vicinity prompted of the city artificial to alleviate breaching, flooding. on This the nighthigh flood of 3 October level 2003,had of been the preceded Langue de by Barbarie several other in the episodes vicinity in of the the 1990s. city to One alleviate function flooding. of the Diama This high dam flood was levelto hadalleviate been preceded floods in bythe severallower valley, other episodesnotably in in the the deltaic 1990s. sector. One functionMietton et of al. the [33] Diama highlighted dam was the to alleviaterather floodsmixed inresults the lowerfrom valley,the flood-control notably in func thetion deltaic of sector.the dam Mietton since the et al.1990s, [33] highlightedand reported the ratherrepeated mixed episodes results fromof severe the flood-controlflooding in St. function Louis in of 1994 the (1.26 dam m since above the IGN 1990s, datum), and reported 1995 (1.21 repeated m), episodes1997 (1.28 of severe m), 1998 flooding (1.43 m), in 1999 St. Louis (1.47 inm), 1994 2001 (1.26 (1.2 mm) aboveand 2003 IGN (1.38 datum), m). The 1995 latter (1.21 events m), 1997 preceding (1.28 m), 1998the (1.43 artificial m), 1999breach (1.47 are m),depicted 2001 (1.2in Figure m) and 7. Th 2003e water (1.38 level m). Theof 1.47 latter m above events IGN preceding datum attained the artificial at breachthe height are depicted of the 1999 in Figure high-flow7. The season water exceeded level of 1.47the 1.2 m abovem flooding IGN datumthreshold attained for 12 atdays, the and height the of theconcern 1999 high-flow voiced by season the exceededpopulation the of 1.2St. m Louis flooding regarding threshold this for flooding 12 days, progressively and the concern brought voiced bypressure the population to bear ofon St.the Louis administrative regarding authorities this flooding in their progressively recourse to brought artificial pressure breaching to bear[34]. onA 4 the administrativem-long and 1.5 authorities m-deep trench in their was recourse cut across to artificiala relatively breaching narrow (100 [34]. m-wide) A 4 m-long portion and of 1.5 the m-deep spit trenchabout was 7 km cut south across of a relativelySt. Louis narrowby engineers (100 m-wide) in the night portion of 3 of October the spit 2003. about This 7 km induced south of a St.rapid Louis byovernight engineers drop in the in night water of level 3 October of up to 2003. 1 m This(Figure induced 7) that a prevented rapid overnight further drop flooding in water [34]. levelFollowing of up to this opening, the trench widened rapidly (Figure 8) and became the new river mouth, a case of 1 m (Figure7) that prevented further flooding [ 34]. Following this opening, the trench widened rapidly inadvertent delta-mouth diversion generated by humans. The width of this artificial breach grew to (Figure8) and became the new river mouth, a case of inadvertent delta-mouth diversion generated by 250 m 3–4 days after the opening. The depth of the breach increased to 6 m by 2007 [28], while the humans. The width of this artificial breach grew to 250 m 3–4 days after the opening. The depth of the width increased to nearly 2 km in October 2006, three years after the breach (Figure 6), before breach increased to 6 m by 2007 [28], while the width increased to nearly 2 km in October 2006, three decreasing once more to ~1 km in early 2008. Channelling of the Senegal River flow in the new yearsenlarged after themouth breach led to (Figure closure6), of before the former decreasing natural once mouth more located to ~1 further km in downdrift. early 2008. An Channelling accelerated of thephase Senegal of widening River flow ensued in the afterwards, new enlarged peaking mouth to nearly led to 5.5 closure km between of the former October natural 2012 and mouth June located2013 further(Figure downdrift. 6). Figure An9 summarises accelerated the phase dynamics of widening of the spit ensued and afterwards,river mouth peakingsince the to 2003 nearly artificial 5.5 km betweenbreach. October The rapid 2012 widening and June was 2013 related (Figure to6 an). Figureadditional9 summarises natural breach the dynamics created in of October the spit and2012 riverby mouthoverwash since 500 the m 2003 south artificial of the breach.new mouth. The rapidMuch wideningof the remaining was related spit between to an additional this new opening natural breachand createdthe mouth in October was eroded 2012 by through overwash several 500 other m south washovers of the new that mouth.tended Muchto coalesce, of the widening remaining the spit betweenmouth thisand newsea-intrusion opening andpathways, the mouth as sand was was eroded transported through southward several other by longshore washovers drift. that tended to coalesce, widening the mouth and sea-intrusion pathways, as sand was transported southward by longshore drift.

Water 2017, 9, 357 9 of 17

Water 2017, 9, 357 9 of 17 Water 2017, 9, 357 9 of 17 Water 2017, 9, 357 9 of 17

Figure 7. Maximum water levels in the Senegal River channel at St. Louis from 1999 to 2006. Adapted FigureFigure 7. Maximum 7. Maximum water water levels levels in in the the Senegal Senegal RiverRiver ch channelannel at at St. St. Louis Louis from from 1999 1999 to 2006. to 2006. Adapted Adapted fromFigure [34]. 7. Maximum water levels in the Senegal River channel at St. Louis from 1999 to 2006. Adapted fromfrom [34]. [34]. from [34].

Figure 8. Ground photographs showing the initial trench (4 October 2003), dug on the night of 3 Figure 8. Ground photographs showing the initial trench (4 October 2003), dug on the night of 3 FigureOctoberFigure 8. Ground 8. 2003, Ground across photographs photographs the Langue showing showingde Barbarie the to initial initialallevi trenate trench floodingch (4 (4October of October parts 2003), of2003), St. dug Louis. on dug Thethe on night5 October the of night 3 of October 2003, across the Langue de Barbarie to alleviate flooding of parts of St. Louis. The 5 October 3 October2003October photograph 2003, 2003, across across shows the the Langue Languethe trench dede BarbarieBarbarie considerably to to allevi alleviate wiatedened flooding ﬂooding by river of ofparts and parts of tidal St. of Louis. St.flow Louis. (PhotoThe The5 October credit: 5 October 2003 photograph shows the trench considerably widened by river and tidal flow (Photo credit: 2003Service photograph2003 photograph régional shows de shows l’Hydraulique, the trenchthe trench considerably St. considerablyLouis du widenedSénégal). widened by riverby river and and tidal tidal ﬂow flow (Photo (Photo credit: credit: Service Service régional de l’Hydraulique, St. Louis du Sénégal). régionalService de l’Hydraulique,régional de l’Hydraulique, St. Louis duSt. Louis Séné gal).du Sénégal).

Figure 9. Cont. Water 2017, 9, 357 10 of 17 Water 2017, 9, 357 10 of 17

FigureFigure 9. Assemblage 9. Assemblage from from Google Google Earth Earth images images showing showing changes changes inin thethe LangueLangue dede Barbarie spit spit and and Senegal River mouth between March 2003, prior to the October 2003 artificial breach, and 2015. Senegal River mouth between March 2003, prior to the October 2003 artificial breach, and 2015. Black: Black: Langue de Barbarie spit and beach sand; dark grey: subaerial lower delta plain potentially Langue de Barbarie spit and beach sand; dark grey: subaerial lower delta plain potentially subject to subject to river flooding (including St. Louis); light grey: delta plain seasonally flooded by the river flooding (including St. Louis); light grey: delta plain seasonally flooded by the Senegal River. Senegal River. From 2012 to 2013, rapid wave-induced erosion of the residual spit downdrift of the Frommouth 2012 toled 2013, to considerable rapid wave-induced mouth widening, erosion an increase of the residualin tidal influence spit downdrift within the of lower the mouth Senegal led to considerabledelta, and mouth direct widening,wave attack an of increaseparts of the in de tidallta plain influence hitherto within protected the lower by the Senegal residual delta, spit. and direct wave attack of parts of the delta plain hitherto protected by the residual spit. 4. Discussion 4. DiscussionThe shoreline of the Senegal delta offers an interesting example of strong wave influence on Thedelta shoreline evolution. of Two the clear Senegal manifestations delta offers of anthis interesting strong influence example are ofthe strong absence wave of a notable influence classic on delta deltaic “bulge”, and the presence of a persistent sand spit, the Langue de Barbarie, an extremely evolution. Two clear manifestations of this strong influence are the absence of a notable classic deltaic mobile feature that generates river-mouth diversion. This spit has been subject to repeated past “bulge”, and the presence of a persistent sand spit, the Langue de Barbarie, an extremely mobile breaches, and delta-mouth migration over a total distance of 28–30 km at least since the mid-17th featurecentury. that generatesThe dominant river-mouth natural mode diversion. of behaviour This spit of hasthe beenSenegal subject delta toshoreline repeated is pastthus breaches,one and delta-mouthimprinted by strong migration longshore over transport a total distance of sand generated of 28–30 by km Atlantic at least waves since from the NW mid-17th to N. The century. The dominantSenegal River natural mouth mode is thus of behaviour a fine example of the of Senegal a wave-influenced delta shoreline delta isillustrating thus one imprintedthe relationship by strong longshorebetween transport river-mouth of sand migration, generated spit elongation by Atlantic and waves spit breaching from NW by the to N.river The mouth Senegal [31], although River mouth is thusa simple a fine examplerelationship of abetween wave-influenced these processes delta ca illustratingnnot be expected the relationship because of between the influence river-mouth of migration,fluctuations spit elongation in river discharge and spit and breaching river-mouth by ba ther dynamics river mouth [11]. Whereas [31], although high river a simple discharge relationship and betweenthe formation these processes of river-mouth cannot bars be expected can lead becauseto reduced of sediment the influence bypassing, of fluctuations which affects in riverin turn discharge the river-mouth migration rate and the size of the river-mouth spit [31], reduced discharge at the river and river-mouth bar dynamics [11]. Whereas high river discharge and the formation of river-mouth mouth, tantamount to a decrease in hydraulic efficiency, can lead to bypassing of sediment around bars canthe mouth, lead to thus reduced reducing sediment migration bypassing, [31,35]. Natural which breaches affects of in spits turn barring the river-mouth river mouths migration and tidal rate and theinlets size are of a thecommonly river-mouth cyclic spitprocess [31 ],determined reduced dischargeby a combination at the river of spit mouth, lengthening, tantamount river to a decreasedischarge in hydraulic and river efficiency, hydraulic efficiency, can lead toand bypassing also in many of sediment cases, storm around wave action the mouth, [31,36]. thus reducing migrationThe [31 absence,35]. Natural of breaching breaches between of spits1973 and barring 2003 riverassociated mouths with andthe lengthening tidal inlets of are the a Langue commonly cyclicde process Barbarie determined spit over bythis a combinationperiod constitutes of spit a lengthening,much longer rivertimescale discharge than andpast riverbreaching hydraulic efficiency,timescales and also[27]. The in many reasons cases, for this storm are not wave clear. action They [ 31are,36 unlikely]. to be related to the wave climate, Thewhich absence is devoid of breachingof storms, betweenwhereas breaching 1973 and 2003tends associated to be initiated with by the high lengthening river discharge of the during Langue de Barbariethe spitflood over season. this periodThe longshore constitutes transport a much volu longermes, timescaleof which thanthe period past breaching 1984–2003 timescales may be [27]. considered as representative, fluctuated but presumably were high enough to ensure spit The reasons for this are not clear. They are unlikely to be related to the wave climate, which is devoid elongation, without natural breaching updrift that could have been caused by a decrease in the of storms,alongshore whereas budget. breaching Spit morphometry tends to be initiated(width, de bypth high and river migration discharge range) during as a thecriterion flood for season. The longshoredetermining transport the fraction volumes, of the ofLST which sequestered the period in the 1984–2003 spit, yields may a bevalue considered of 54%. This as representative, value is fluctuated but presumably were high enough to ensure spit elongation, without natural breaching updrift that could have been caused by a decrease in the alongshore budget. Spit morphometry (width, depth and migration range) as a criterion for determining the fraction of the LST sequestered in the spit, yields a value of 54%. This value is moderate relative to the relatively high hindcast and predicted Water 2017, 9, 357 11 of 17 values of the sediment bypassing fraction, β [31] (respectively, 0.83 and 0.74, 1 representing 100% bypassing) for the Senegal River mouth. These rates are, however, quite similar to those (0.8–0.9) calculated from our data on spit morphometry and LST using the sediment bypassing fraction equation and 50% of the river mouth depth as an estimate of the “updrift sediment spit depth” (see Materials and Methods). The mouth of the Senegal has thus been characterised by moderate to high bypassing that assured a degree of growth of the Langue de Barbarie but also the stability of the barrier and coast downdrift of the 2003 artificial breach. The absence of breaching over this long phase has been attributed to a decrease in river discharge [37]. Unfortunately, there are no available data on river water discharge to enable us to tie up natural breaches with the hydraulic efficiency of the river mouth. Mietton et al. [33] noted a total absence of critical floods between 1974 and 1993 associated with the Sahelian drought. This period also incorporates the construction of the Diama dam in 1986. While the breaching timescale since 1973 appears exceptional compared to the pre-1973 conditions, the breaching length is also an important parameter in the onset of breaching [31]. The elevation of the water surface at the upstream boundary of a river channel is directly related to the channel length, such that an increase in the latter, as the river mouth migrates, results in a constant water surface slope, with the eventuality of breaching when a critical channel length is attained [31]. Guilcher [32] and Gac et al. [27] reported that the Langue de Barbarie spit generally did not exceed a maximum length of 28–30 km, beyond which breaching tended to occur. This length probably corresponds to the breaching length defined by Nienhuis et al. [31]. There is a probability, therefore, that a natural breach could have been imminent close to the time frame of the 2003 artificial breach. A reason for advancing this hypothesis is the increase in flooding (Figure7), which suggests increasing impoundment of flood waters over the lower delta plain and decreasing hydraulic efficiency of the mouth. Whereas natural breaching has been a characteristic of the spit, spit instability since 2003 reflects, in part, the consequences of hasty artificial breaching to solve an impending flooding problem facing St. Louis. By protecting St. Louis and numerous smaller settlements and agricultural land within the delta plain from waves and marine influence, the spit is a major feature of the dynamics and management of the Senegal delta shoreline. Paradoxically, by impounding flood waters of the Senegal River, the spit also contributes to a flood risk that has grown apace with the urban extension of St. Louis. The long phase of absence of breaching between 1973 and 2003 coincided with a period of rapid tourism development in the Senegal delta associated with the emplacement of tourist infrastructure on the rectilinear spit that provided sandy grounds well above flood level. Although much of the lower delta is characterised by a population density of only about ten inhabitants/km2, there are zones of very high population concentrations, as in St. Louis and certain areas of the Langue de Barbarie such as Guet-Ndar (Figure9) where the 2013 census shows densities exceeding 80,000 inhabitants/km2 [6]. The artificial breach annihilated the risk of flooding of St. Louis in 2003 and in the following years by enabling more rapid seaward drainage of river water during the high-flow season [34]. As in the pre-2003 period, the sediment bypassing fraction, β [31], across the mouth of the Senegal River has been quite high (0.8–0.9), although balancing spit morphometry against LST over the same period suggests up to 40% of sand locked up in spit growth, a value lower, however, than that of the pre-2003 breach. There have been marked fluctuations in spit growth, however, with even spit erosion in 2005–2006, 2008–2010 and 2012–2013. Under conditions of spit growth, sand has been incorporated in new recurves that mark the current form of elongation of the residual updrift spit sector, which is also characterised by an enlarged distal tip (Figure9). The reasons for alternations between spit growth (including widening) and spit erosion are not clear. They may be related to variations in higher-energy waves, and potentially varying LST, as shown by the drop in the number of days with high-energy waves in 2012 (Figure 10) and the correlative drop in LST (Figure4), but they could also be an outcome of variability in river discharge and sediment bypassing. Water 2017, 9, 357 12 of 17 Water 2017, 9, 357 12 of 17

FigureFigure 10.10.Significant Significantheights heights ( Hs(Hs)) of of high-energy high-energy waves waves ( ±(±1.61.6 standardstandard deviationsdeviations aroundaround meanmean HsHs)) fromfrom 1999 1999 to to 2015 2015 (top (top); and); and number number of daysof days per per year year with with high-energy high-energy waves waves along along the Senegal the Senegal River deltaRiver coast, delta derivedcoast, derived from ERA from hindcast ERA hindcast data (bottom data (bottom). Orange:). Orange: swell waves swell (wavesHs ≥ 2.37 (Hs m),≥ 2.37 blue: m), wind blue: waveswind (wavesHs ≥ 1.36 (Hs m).≥ 1.36 Note m). the Note significant the significant drop in high-energydrop in high-energy swell waves swell in 2012waves (see in also2012 Figure (see also4). Figure 4). Over this post-2003 period, fluctuations of the width of the river mouth (Figure6) are presumably a functionOver ofthis the post-2003 balance between period, the fluctuations river’s hydraulic of the efficiency,width of includingthe river the mouth tidal discharge,(Figure 6) andare incidentpresumably wave a energyfunction and of sedimentthe balance bypassing between [ 11the]. Theriver’s width hydraulic of the “fluvial”efficiency, mouth including of the the river tidal is verydischarge, likely inand the incident range of wave ~0.5–1 energy km, whichand sediment is the “usual” bypassing mouth [11]. width The width (Figure of6 )the and “fluvial” the stabilised mouth widthof the attainedriver is very shortly likely after in thethe artificialrange of breach.~0.5–1 km, The which rapid wideningis the “usual” between mouth October width 2012(Figure and 6) June and 2013the stabilised occurred width following attained wave shortly overwash after and the erosionartificial of breach. the remaining The rapid spit widening downdrift between of the October mouth. This2012 rapidand June erosion 2013 would occurred appear following to result wave from overwash a combination and erosion of the of mostthe remaining significant spit drop downdrift in LST recordedof the mouth. (2010–2012) This rapid over erosion the period would 1984–2015 appear to (Figure result 4from), with a combination a lag effect inof time,the most and significant possible sequesteringdrop in LST ofrecorded sand in (2010–2012) the river mouth. over Lowerthe period bypassing 1984–2015 (due (Figure to higher 4), river with discharge?)a lag effect andin time, a sharp and increasepossible in sequestering LST from 2012 of sand to 2013 in (anthe increaseriver mouth. of about Lower ~45% bypassing relative to(due the to 2010–2012 higher river LST discharge?) (Figure4)) couldand a explainsharp increase the ensuing in LST exceptionally from 2012 to rapid 2013 elongation(an increase of of the about Langue ~45% de relative Barbarie to the spit 2010–2012 between JuneLST 2013(Figure and 4)) May could 2015 explain (~2 km) the (Figure ensuing9). exceptiona A reviewlly of conceptualrapid elongation advances of the in wave-river-mouthLangue de Barbarie interactionsspit between [11 June] and 2013 modelling and May of alongshore 2015 (~2 km) sediment (Figure bypassing 9). A review at river of mouths conceptual [31] haveadvances shown in thatwave-river-mouth waves refracting interactions over the river-mouth [11] and modelling bar create aof zone alongshore of low alongshore sediment sedimentbypassing transport at river updriftmouths which [31] have reduces shown sediment that waves bypassing. refracting These over observations the river-mouth imply that bar the create LST potentiala zone of south low ofalongshore the new mouthsediment is beingtransport assured updrift by which a degree redu ofces “cannibalisation” sediment bypassing. of the These rest of observations the spit, as imply sand transportedthat the LST frompotential the northsouth hasof the been new increasingly mouth is being trapped assured updrift by a ofdegree the wider of “cannibalisation” mouth, presumably of the leadingrest of the to lowerspit, as bypassing. sand transported Except from for 2007–2009,the north has and been 2010–2011, increasingly this trapped sector has updrift been of in the erosion. wider Thismouth, demise presumably of the spit leading downdrift to lower of the bypassing. new mouth Except has for led 2007–2009, to the destruction and 2010–2011, of villages, this campsitessector has andbeen other in erosion. tourist This structures. demise The of the delta spit plain downdr in thisift erodingof the new sector mouth is now has directly led to exposedthe destruction to ocean of wavesvillages, and campsites erosion thatand areother threatening tourist structures. numerous The villages. delta plain in this eroding sector is now directly exposedMuch to ofocean the lowerwaves delta and erosion plain and that the are main threatening river channel numerous are now villages. situated over 20 km upstream of theMuch former of mouth,the lower between delta plain the newand the mouth main and river the channel anti-salt are intrusion now situated Diama over dam 20 thatkm upstream confines theof the tidal former prism mouth, to the lowerbetween delta the plain.new mouth In consequence, and the anti-salt the much intrusion wider Diama mouth dam appears that confines to have becomethe tidal favourable prism to tothe a largerlower tidaldelta prism, plain. manifested In consequence, by an increasethe much in thewider tidal mouth range inappears St. Louis, to have and confirmedbecome favourable by recent to studies a larger [33 ,tidal34]. Durandprism, manifested et al. [34] showed by an increase that the in maximum the tidal semi-diurnalrange in St. Louis, tidal rangeand confirmed downstream by ofrecent the Diamastudies dam [33,34]. has increasedDurand et three-fold, al. [34] showed from a that mean the of maximum 0.30 m in 2001–2002semi-diurnal to tidal range downstream of the Diama dam has increased three-fold, from a mean of 0.30 m in 2001– 2002 to 0.93 m in 2004–2005, whereas the mean maximum spring tide range attained 1.18 m, for a

Water 2017, 9, 357 13 of 17

0.93 m in 2004–2005, whereas the mean maximum spring tide range attained 1.18 m, for a predicted value of 1.29 m, along the Langue de Barbarie spit. These authors have also noted that the semi-diurnal tidal effects are now more clearly expressed even during the high river flood waters. The impacts of these changes are still to be studied, but it may be expected that they are leading to increasing soil salinization in the lower delta plain, to the extension of bare saline flats, and to modifications in biodiversity. The extent to which accelerated subsidence, one of the two major causes of delta vulnerability (together with rapid and chronic erosion), affects the delta is not known, although it may be inferred that a decreasing sediment load and damming may be contributing to more exacerbated flooding in the delta plain. However, the problem seems to have more to do with accelerated urbanisation of St. Louis over the last few decades, bringing new populations to encroach on areas of the delta that are susceptible to flooding during exceptionally wet years. Durand et al. [34] have highlighted the potential vulnerability of the city and the surrounding low delta plain to sea-level rise. Their model simulating flood propagation in the city, and based on various sea-level scenarios, shows the susceptibility of St. Louis to flooding during the highest annual water levels in the course of the 21st century.

5. Materials and Methods

5.1. Waves and Wave-Induced Longshore Transport In order to estimate the wave-induced alongshore transport on the Langue de Barbarie, we extracted bulk wave parameters (signiﬁcant height Hs, peak period Tp and direction of both swell and wind waves) from hindcast data in the Atlantic Ocean between 1984 and 2015, generated by the ECMWF Wave Atmospheric Model (WAM) model [38]. The wave data are part of the ERA-Interim dataset, which involves a reanalysis of global meteorological variables [39,40]. Wave data were extracted from the ECMWF data server on a 0.5◦ × 0.5◦ grid, with a 6-h temporal resolution and covering the sector 16.5◦ N/17◦ W. The ERA-40 and the following ERA-Interim reanalysis are the ﬁrst in which an ocean wind–wave model is coupled to the atmosphere, and the quality of the wave data has been extensively validated against buoy and altimeter data. Sterl and Caires (2005) [40] demonstrated a very good correlation between the ERA-40 data and these sources, except for high waves (Hs > 5 m) and low waves (Hs < 1 m), which tend, respectively, to be under- and over-estimated [41]. These critical wave conditions are not typical of the relatively constant wave regime affecting the Senegal delta coast, and extreme wave condition issues reported for ERA-40 are partially resolved for higher resolution ERA-Interim. However, the Senegal coast has scarce observations, and this affects the hindcast quality. ERA-40 and -Interim results in this region should be taken with caution. Several alongshore sediment transport formulae exist and are widely applied by coastal engineers and dynamicists. However, there is still an important research effort on the improvement of alongshore sediment transport parameters and no large consensus on the choice of a formulation, as dispersion between predictors is often substantial [42], and validation dataset at the regional scale scarce. Here, we chose the formula of Kaczmarek et al. [23] because of its straightforward implementation for remote sites such as the Langue de Barbarie where only limited observations exist and because it has been applied to similar environments [43,44]. The amount of sediment drifting alongshore was computed as follows: 2 2 Q = 0.023 Hb V i f dbV < 0.15 (1)

2 2 Q = 0.00225 + 0.008 Hb V i f dbV > 0.15 (2) where Hb is the breaking wave height and V an estimation of the alongshore current within the surf zone derived from the commonly used formula of Longuet-Higgins [45]: p V = 0.25kv γgdb sin2αb (3) Water 2017, 9, 357 14 of 17

where αb is the local breaking wave angle, γ = Hb/db = 0.78 is the breaker parameter constant [46], g the 2 gravitational acceleration (m/s ), Hb the breaking wave height, db the local water depth and kv an empirical constant. Here, we used kv = 2.9 based on the values of Bertin et al. [43] for wave-dominated environments with similar grain-size characteristics. A separate computation for sediment transport induced, respectively, by wind waves and swell waves was conducted. Alongshore sediment transport formulae necessitate breaking wave parameters as inputs, but global wave hindcast only provide deepwater characteristics. While a nested model (e.g., SWAN or WW3) to propagate waves from deepwater to the breakpoint would be ideal for a short-term study, the present analysis focuses on seasonal to inter-annual wave variations covering a long period of 32 years. We chose therefore to use the direct breaking wave predictor proposed by Larson et al. [47]. This formula provides breaking wave height Hb and angle αb from deepwater wave height Ho, period T and incidence angle α0: 2 Hb = λC /g (4) √ αb = asin sin(α0) λ (5) with a correction factor λ computed as: λ = ∆λa (6) considering ∆ = 1 + 0.1649 ξ + 0.5948 ξ2 − 1.6787ξ3 + 2.8573 ξ4 (7) !4 2 2/5 C C 2 ξ = λasinθ0 , λa = [cos(α0)/θ] , θ = p γ (8) gH Cg where deep water phase celerity is given by C = 1.56T, wavelength L = 1.56T2, and group celerity Cg = C/2.

5.2. Shoreline Change and Spit and River-Mouth Dynamics In order to highlight recent deltaic shoreline changes, we resorted to available aerial photographs (1954), a CORONA satellite image (1968) and LANDSAT (1984–1988, 1992, 1999–2004, 2006–2011, 2013, 2015–2016) and SPOT satellite images (2005) with moderate pixel size resolution (30 to 60 m) made available by the USGS and the French IGN. The main items analysed were spit length and corresponding migration rates, spit width, and river-mouth width and the underlying dynamics. The spatial data were chosen to cover the entire “delta-inﬂuenced” shoreline for each year of analysis and with a cloud cover not exceeding 10%. We limited our choice to images taken at low tide and systematically in January of every year to minimise seasonal and tidal distortions (tides induce very little variability in the microtidal context of the Senegal River delta). The results on shoreline change were completed by a literature review on the past dynamics of the Langue de Barbarie and by ﬁeld observations of this spit conducted in 2005, 2007 and 2016. Based on data from the satellite images and aerial photographs on spit and river-mouth characteristics, the fraction of sediment bypassing the mouth, β, assuming conservation of mass, was inferred from the following relationship [31]:

ν = Qs(1 − β)/Ab (9) where ν is the migration rate of the mouth (m·s−1), Qs is the volumetric alongshore sediment transport rate (m3·s−1), and Ab = Ws·Ds which is the cross-sectional area of the river mouth spit (m2) composed of blocked littoral sediment from the updrift coast, Ws the width of the spit, and Ds spit updrift sediment depth. Water 2017, 9, 357 15 of 17

Acknowledgments: This work benefited from the ECMWF ERA Interim dataset (www.ECMWF.Int/research/Era), and from Landsat satellite images provided by the United States Geological Survey, and Spot satellite images provided by Centre National d’Etudes Spatiales. We acknowledge funding from the Belmont Forum Project: BF-Deltas: Catalyzing Action Towards Sustainability of Deltaic Systems with an Integrated Modeling Framework for Risk Assessment. Mamadou Sadio benefited from a partial PhD grant provided by the Embassy of France in Senegal. We thank the anonymous reviewers for their salient suggestions for improvement. Author Contributions: Mamadou Sadio, Edward J. Anthony, Amadou Tahirou Diaw and Alioune Kane designed the project. Mamadou Sadio, Amadou Tahirou Diaw, Philippe Dussouillez and Jules T. Fleury analysed the satellite images. Mamadou Sadio., Edward J. Anthony, Amadou Tahirou Diaw, and Alioune Kane conducted field reconnaissance. Rafael Almar and Elodie Kestenare analysed the E.R.A. data and longshore transport products. All authors wrote the paper. Conflicts of Interest: We declare no conflicts of interests.

References

1. Crossland, C.J.; Kremer, H.H.; Lindeboom, H.J.; Marshall Crossland, J.I.; Le Tissier, M.D.A. Coastal Fluxes in the Anthropocene; Springer: Berlin, Germany, 2007; p. 231. 2. Van Rijn, L.C. Coastal erosion and control. Ocean Coast. Manag. 2011, 54, 867–887. [CrossRef] 3. Davies, J.L. Geographical Variation in Coastal Development, 2nd ed.; Longman: London, UK, 1980; p. 212. 4. Anthony, E.J. Coastal progradation in response to variations in sediment supply, wave energy and tidal range: Examples from Sierra Leone, West Africa. Géodynamique 1991, 6, 57–70. 5. Anthony, E.J. The muddy tropical coast of West Africa from Sierra Leone to Guinea-Bissau: Geological heritage, geomorphology and sediment dynamics. Afr. Geosci. Rev. 2006, 13, 227–237. 6. Diatta, I. L’ouverture d’une Brèche à Travers la Langue de Barbarie (Saint-Louis du Sénégal). Les Autorités Publiques et les Conséquences de la Rupture. Master’s Thesis, Université Gaston Berger, St Louis, Senegal, 2004.; p. 116. 7. Sy, B.A. L’ouverture de la brèche sur la Langue de Barbarie et ses conséquences. Approche géomorphologique. Revue de Géographie de Saint-Louis 2004, 4, 50–60. (In French) 8. Bhattacharya, J.P.; Giosan, L. Wave-inﬂuenced deltas: Geomorphological implications for facies reconstruction. Sedimentology 2003, 50, 187–210. [CrossRef] 9. Coleman, J.M.; Huh, O.K. Major Deltas of the World: A Perspective from Space; Coastal Studies Institute, Louisiana State University: Baton Rouge, LA, USA, 2004. 10. Anthony, E.J. Patterns of sand spit development and their management implications on deltaic, drift-aligned coasts: The cases of the Senegal and Volta River delta spits, West Africa. In Sand and Gravel Spits; Randazzo, G., Cooper, J.A.G., Eds.; Springer: Berlin, Germany, 2015; Volume 12, pp. 21–36. 11. Anthony, E.J. Wave inﬂuence in the construction, shaping and destruction of river deltas: A review. Mar. Geol. 2015, 361, 53–78. [CrossRef] 12. Nienhuis, J.H.; Ashton, A.D.; Giosan, L. What makes a delta wave-dominated? Geology 2015, 43, 511–514. [CrossRef] 13. Galloway, W.E. Process framework for describing the morphologic and stratigraphic evolution of delta depositional systems. In Deltas: Models for Exploration; Broussard, M.L., Ed.; Texas Geological Society: Houston, TX, USA, 1975; pp. 87–98. 14. Sall, M. Crue et Elévation du Niveau Marin à Saint-Louis du Sénégal: Impacts Potentiels et Mesures D’adaptation. Ph.D. Thesis, Université du Maine, Le Mans, France, 2006. 15. Kamara, S.; Martin, Ph.; Coly, A. Organisation traditionnelle du bas delta du Sénégal et nouvelles régulations hydrauliques. Dimension anthropospatiale d’un développement. Revue Espaces et Sociétés en Mutation 2015, 2015, 127–144. (In French). 16. Mahé, G.; Olivry, J.C. Variations des précipitations et des écoulements en Afrique de l’Ouest et central de 1951 à 1989. Sécheresse 1995, 6, 109–117. (In French). 17. Kane, A.; Niang-Fall, A. Hydrologie du Sénégal; Atlas Jeune Afrique: Dakar, Sénégal, 2007; p. 14. (In French) 18. Ostenfeld, C.; Jonson, N. Etude de la Navigabilité et des Ports du Fleuve Sénégal; Études Portuaires à Saint-Louis, Kayes et Ambidebi. Vol. 1: Travaux Préliminaires; Vol. 2, Annexe 2: Rapport Sur les Enquêtes Hydrauliques; Surveyer-Nenninger et Chevenert Inc.: Montréal, QC, Canada, 1972. (In French) Water 2017, 9, 357 16 of 17

19. Gac, J.Y.; Kane, A. Le fleuve Sénégal. Bilan hydrique et flux continentaux de matières particulaires à l’embouchure. Sci. Geol. 1986, 39, 99–130 & 151–172. (In French). 20. Food and Agriculture Organization of the United Nations (FAO). Caractérisation Des Systèmes de Production Agricole au Senegal; Document de Synthese; FAO: Rome, Italy, 2007. (In French) 21. Michel, P. The southwestern Sahara margin: Sediments and climate change during the recent Quaternary. Palaeoecol. Afr. Surround. Isl. 1980, 12, 297–306. 22. Monteillet, J. Environnements Sédimentaires et Paléohcologie du Delta du Sénégal au Quaternaire; Lmprimerie des Tilleuls: Millau, France, 1986; p. 267. (In French) 23. Kaczmarek, L.M.; Ostrowski, R.; Pruszak, Z.; Rozynski, G. Selected problems of sediment transport and morphodynamics of a multi-bar nearshore zone. Estuar. Coast. Shelf Sci. 2005, 62, 415–425. [CrossRef] 24. SOGREAH. Etudes de Faisabilité et D’avant Projet Sommaire de L’émissaire Delta; Rapport Final: Grenoble, France, 1994; p. 70. (In French) 25. Barusseau, J.P.; Bâ, M.; Descamps, C.; Diop, E.S.; Diouf, B.; Kane, A.; Saos, J.L.; Soumaré, A. Morphological and sedimentological changes in the Senegal River estuary after the constuction of the Diama dam. J. Afr. Earth Sci. 1998, 26, 317–326. [CrossRef] 26. Sall, M.M. Dynamique et Morphogenèse Actuelles au Sénégal Occidental. Ph.D. Thesis, Université Louis Pasteur-Strasbourg I, Strasbourg, France, 1982. 27. Gac, J.Y.; Kane, A.; Monteillet, J. Migrations de l’embouchure du fleuve Sénégal depuis 1850. Cahiers ORSTOM Série Géologie 1982, 12, 73–76. (In French). 28. Bâ, K.; Wade, S.; Niang, I.; Trébossen, H.; Rudant, J.P. Cartographie radar en zone côtière à l’aide d’images multidates RSO d’Ers-2: Application au suivi environnemental de la Langue de Barbarie et de l’estuaire du fleuve Sénégal. Télédétection 2007, 7, 129–141. (In French). 29. Joiré, J. Amas de coquillages du littoral sénégalais dans la banlieu de Saint-Louis. Bulletin de l’Institut Français de l’Afrique Noire 1947, 9, 170–340. (In French). 30. Tricart, J. Notice Explicative de la Carte Géomorphologique du Delta du Sénégal;Mémoires, B.R.G.M., Ed.; Bureau de Recherches Geologiques et Minieres: Orléans, France, 1961; Volume 8, p. 137. (In French) 31. Nienhuis, J.H.; Ashton, A.D.; Nardin, W.; Fagherazzi, S.; Giosan, L. Alongshore sediment bypassing as a control on river mouth morphodynamics. J. Geophys. Res. Earth Surf. 2016, 121, 664–683. [CrossRef] 32. Guilcher, A.; Nicholas, J.P. Observation sur la Langue de Barbarie et les bras du Sénégal aux environs de Saint-Louis. Bulletin d’Information du Comité Océanographique pour les Etudes Côtières 1954, 6, 227–242. (In French) 33. Mietton, M.; Dumas, D.; Hamerlynck, O.; Kane, A.; Coly, A.; Duvail, S.; Baba, M.L.O.; Daddah, M. Le delta du fleuve Sénégal. Une gestion de l’eau dans l’incertitude chronique. In Incertitudes et Environnement—Mesures, Modèles, Gestion; d’Allard, P., Denis, F., Picon, B., Eds.; Ecologie Humaine/Edisud: Arles, France, 2006; pp. 321–336. (In French) 34. Durand, P.; Anselme, B.; Thomas, Y.F. L’impact de l’ouverture de la brèche dans la langue de Barbarie à Saint-Louis du Sénégal en 2003: Un changement de nature de l’aléa inondation? Cybergeo 2010, 496. (In French) [CrossRef] 35. Balouin, Y.; Ciavola, P.; Michel, D. Support of subtidal tracer studies to quantify the complex morphodynamics of a river outlet: The Bevano, NE Italy. J. Coast. Res. 2006, 39, 602–606. 36. Cooper, J.A.G. Ephemeral stream-mouth bars at flood-breach river mouths on a wave dominated coast: Comparison with ebb-tidal deltas at barrier inlets. Mar. Geol. 1990, 95, 57–70. 37. Niang, A.J. Les Processus Morphodynamiques, Indicateurs de L’état de la Désertification Dans le Sud-Ouest de la MAURITANIE. Approche Par Analyse Multisource. Ph.D. Thesis, Université de Liège, Liège, Belgium, 2008. 38. The Wamdi Group. The WAM model—A third generation ocean wave prediction model. J. Phys. Oceanogr. 1988, 18, 1775–1810. 39. Dee, D.P.; Uppala, S.M.; Simmons, A.J.; Berrisford, P.; Poli, P.; Kobayashi, S.; Andrae, U.; Balmaseda, M.A.; Balsamo, G.; Bauer, P.; et al. The ERA-interim reanalysis: Configuration and performance of the data assimilation system. Q. J. R. Meteorol. Soc. Bull. 2011, 137, 553–597. [CrossRef] 40. Sterl, A.; Caires, S. Climatology, variability and extrema of ocean waves—The web-based KNMI/ERA-40 Wave Atlas. Int. J. Climatol. 2005, 25, 963–977. [CrossRef] Water 2017, 9, 357 17 of 17

41. Caires, S.; Swail, V.R.; Wang, X.L. Projection and analysis of extreme wave climate. J. Clim. 2006, 19, 5581–5605. [CrossRef] 42. Pinto, L.; Fortunato, A.B.; Freire, P. Sensitivity analysis of non-cohesive sediment transport formulae. Cont. Shelf Res. 2006, 26, 1826–1839. [CrossRef] 43. Bertin, X.; Castelle, B.; Chaumillon, E.; Butel, R.; Quique, R. Alongshore drift estimation and inter-annual variability at a high-energy dissipative beach: St. Trojan Beach, SW Oleron Island, France. Cont. Shelf Res. 2008, 28, 1316–1332. [CrossRef] 44. Almar, R.; Kestenare, E.; Reyns, J.; Jouanno, J.; Anthony, E.J.; Laibi, R.; Hemer, M.; Du Penhoat, Y.; Ranasinghe, R. Part 1. Wave climate variability and trends in the Gulf of Guinea, West Africa, and consequences for longshore sediment transport. Cont. Shelf Res. 2015, 110, 48–59. [CrossRef] 45. Longuet-Higgins, M.S. Alongshore currents generated by obliquely incident sea waves. J. Geophys. Res. 1970, 75, 6788–6801. 46. Battjes, J.A.; Janssen, J.P.F.M. Energy loss and setup due to breaking of random waves. In Proceedings of the ASCE International Conference on Coastal Engineering, Hamburg, Germany, 27 August–3 September 1978; pp. 569–587. 47. Larson, M.; Hoan, L.X.; Hanson, H. A direct formula to compute wave properties at incipient breaking. J. Waterw. Port Coast. Ocean Eng. 2010, 136, 119–122. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).